1. Field of the Invention
This invention relates to piezoelectric/pyroelectric elements and a novel process for producing these elements.
2. Description of the Related Art
Piezoelectric materials are materials that produce an electric voltage when subjected to mechanical strain or in the reverse are mechanically deformed when they are subjected to an electric field. Pyroelectric materials are those materials which produce an electric voltage when heated.
This electric voltage is normally collected from the piezoelectric or pyroelectric material via a pair of electrodes connected to conducting wires and mounted upon two relevant faces of the material. In a reverse manner an electric field may be applied utilizing the conducting wires connected to the electrode pair.
It is now known that certain plastics substances can be made into films or sheets having a crystalline form which can be treated so as to become strongly piezoelectric or pyroelectric. A search of the literature will show that many polymers have been identified as potential piezoelectric or pyroelectric materials. One particular polymer is poly(vinyl difluoride), also known as polyvinylidene fluoride and often referred to as PVDF. Thin films of PVDF can be made strongly piezo- or pyroelectric by a process called poling in which the thin films are polarized. In this process a thin film is heated to near the softening temperature under tension, placed in a strong polarizing electric field, and allowed to cool slowly to room temperature while maintaining the electric field.
Piezoelectric and pyroelectric elements made from polarized polymers such as PVDF are of considerable interest in a number of fields. For example, a power generating plant could be designed using ocean waves for mechanical deformation of a large surface area of piezoelectric elements. Another use for piezoelectric elements is as transducers.
For some applications the sensitivity of a fully polarized polymeric piezoelectric/pyroelectric element is at least in theory increased with increasing film thickness. However due to thermally-activated electrical breakdown during the poling step of the polymeric film, extremely thick fully polarized polymeric films are difficult to produce and may not be thick enough to give the desired sensitivity.
U.S. Pat. No. 4,405,402 to Quilliam discloses a piezoelectric/pyroelectric element of an electrically polarized laminar stack of thin polarizable plastic films, each film essentially in face to face contact with its two neighboring films and the poling operation performed after assembling the individual plastic films.
U.S. Pat. No. 4,330,730 to Kurz et. al. disclose a wound piezoelectric material multilayered structure achieved by winding a flexible piezoelectric material film about an axis to produce a plurality of generally concentric piezoelectric layers. Both sides of the wound piezoelectric material film may be provided with conductive coatings which serve as electrodes prior to winding.
Multiple layer piezoelectric elements are attractive because of the large interfacial surface area of piezoelectric material from which generated charges can be collected.
However, the generation of a large number of layers of piezoelectric material is hardly possible without major secondary operations, such as lamination of individual plastic films or folding and winding a film about an axis to produce multiple layers.
The capability to produce relatively thin individual layers of piezoelectric/pyroelectric material having a consistant and uniform thickness, as thin as about 500 Angstroms (0.05 micron) also increases the efficiency of the poling operation through the use of the lower voltage to affect polarization and a shorter time period to achieve polarization.
Another problem with multilayer piezoelectric/pyroelectric elements, especially those produced by laminating individual plastic films, is the ease of establishing junction points so as to not produce a short-circuit.
Therefore, it would be highly desirable to provide a piezoelectric/pyroelectric element having multiple and relatively thin individual layers of piezoelectric/pyroelectric material having easily established junction points and a method for manufacturing the same.